Interest has been focused in recent years on digital transmission systems wherein both voice and data could be accommodated. For example, U.S. Pat. No. 3,549,814 issued to R. J. Jaeger, Jr. et al on Dec. 22, 1970 discloses a pulse code modulation (PCM) multiplexing system where all but one of the digit spaces in each of the 24 channels of the system are used to transmit voice frequency message signals and that digit is used to transmit its order of a digital word. In this manner, a 24 channel system can transmit 24 voice frequency signals and a 24 bit digital word. Another technique is disclosed in U.S. Pat. No. 4,053,712 issued to A. Reindl on Oct. 11, 1977 wherein an adaptive digital coder and decoder is described wherein the coder monitors the idle pattern code generated by the coder and introduces special code words into inter-syllable and inter-word pauses of normally encoded speech signals.
Various encoding techniques have been used to encode analog signals into digital signals for subsequent transmission. One such technique is with the use of an adaptive algorithm for expanding or contracting the encoding range as the analog signal amplitude expands or contracts, respectively. One such technique is the adaptive delta modulation which is a pulse modulation technique in which a continuous analog signal is adaptively converted into a digital pulse pattern for transmission through a channel which can be of low quality, as is well known in the art.
The problem remaining in the prior art is to provide a digital communication system which uses an adaptive algorithm for coding and decoding which can also accommodate short asynchronous data bursts without upsetting the adaptive algorithm states at both ends of the communication channel.